Lead is often used as a shielding material in radiation evaluation equipment in order to reduce the system background radiation. Lead, however, contains small amounts of radioactive isotopes including lead-210, bismuth-210 and polonium-210.
In electronic devices, lead and lead alloys are often used in contacts and solder pads. Integrated circuit memories can suffer from soft errors that can destroy the data in a memory cell and are caused by the alpha particles emitted from the decay daughters of Pb-210, particularly Po-210. Pb 210 has a half-life of 22 years.
Po-210 is well-known as a source of alpha particle emission and it is, therefore, of prime importance to use a lead that has a low alpha particle emission, especially in the above-mentioned applications. The emission is usually measured as a count expressed in the number of alpha particles emitted per cm.sup.2 per hour (alpha count hereinafter). Commercially available lead has alpha counts that may vary from as low as 0.25 to as high as 10 and, unless each batch of lead is analyzed for its alpha count, there is no method for predicting which commercial lead has a low count. There is no commercial process known whereby the Pb 210 can be easily removed from commercial lead. Japanese Patent 59-64 791 describes producing a low alpha lead, containing &lt;50 ppb radio isotopes and an alpha count of .ltoreq.0.5, by electrolyzing a sulfamic acid-lead electrolyte using a lead anode. In spite of the fact that Pb-210 has a half life of 22 years, even lead that is several hundred years old, such as recovered from sunken ships or from church roofs in Europe, has counts of 0.03 to 0.07. These alpha counts are much higher than the level required for electronic devices and integrated circuits. The desired alpha count in the electronics industry is 0.02 or less.
Zone refining, which is a successful method for removing substances that emit alpha particles (alpha emitters hereinafter) from aluminum, does not remove Pb-210 from lead. Although a temporary decrease in alpha count is obtained when lead is zone refined with the initial removal of Bi-210 and Po-210, the count increases again with time to its original level as secular equilibrium is regained, indicating that Pb-210 is not removed.